Exactly. Variance in amplitude. When you hear both parties of a conversation the volume remains fairly constant and your brain can put the conversation in background and you can pretty much ignore it (unless someone mentions your name). A one sided conversation is constantly flipping between low and high volume, and thus constantly grabbing your attention, also, since you only hear one part, a bit of your consciousness is enlisted to figure out what they're talking about. One solution: you provide the unheard part of the conversation: annoys the **** out of the inconsiderate cel phoner.